Measurement of quantities of an electrolytically-active species in aqueous solution previously has been done by a variety of analytical techniques. These techniques have had various respective drawbacks.
Controlled potential coulometric analysis is one method in this respect. As explained, for example, in U.S. Pat. No. 4,244,800, controlled potential coulometry is a method of measuring the quantity of a particular electrolytically-active species in a solution by carrying out an electrochemical reaction involving the electrolytically-active species to be measured. The reaction chosen involves the passage of an electric current and knowledge of the oxidation states of the reactants. The amount of current that flows while the reaction proceeds to a determinable fraction of completion provides a measure of the quantity of the substance in solution. The controlled potential coulometric method typically is carried out using a controlled potential coulometer with a three electrode configuration. The device typically is used for controlling the potential of a working electrode to a selected potential with respect to a reference electrode by applying enough voltage and passing enough current between the working electrode and a counter electrode to cause this selected potential to be maintained. The value of the control potential is selected to favor the particular reaction that is desired and thus to discriminate against unwanted reactions. As also explained in the indicated U.S. Pat. No. 4,244,800, several problems generally arise in the making of precise and accurate measurements by conventional controlled potential coulometry. The first is the fact that to permit the reaction to proceed substantially to completion often takes an appreciable amount of time using conventional analysis arrangements. The lengthy analysis time also leads to another problem of increased chance of changes in parameters, such as temperature, during the measurement period that can produce error in the readings. To reduce analysis time, an empirical end-point method may be used, which refers to a technique in which the analysis is terminated at what is believed to be a predetermined fraction of the final value. Methods of coulometry previously used in which the reaction is not carried to completion have possibilities for error that are unacceptable for highly accurate quantitative measurement of substances in solution.
Chemical titration is another analytical technique for determining quantities of an electrolytically-active species in aqueous solution. For example, on-line chemical titration equipment is commercially available, which feasibly could be adapted for use with an existing iodometric method, which involves the following reactions:NH2Cl+3I−+2H+→NH4++Cl−+I3−2S2O3=+I3−→S4O6=+3I−.
Two solutions are required for this iodometric determination (i.e., acidified KI and sodium thiosulfate titrant), and the supplies of these solutions at the analysis location would have to be periodically replenished to continue use of the analysis. Furthermore, the acidified KI solution is subject to oxidation by atmospheric oxygen, so its shelf life is very limited. These requirements can lead to a significant maintenance burden for the operators.
On-line equipment for colorimetric analyses is available from Hach Co. (Loveland, Colo. U.S.A.) and other equipment suppliers. One type of colorimetric test that can be applied by these types of measurement devices involves the known DPD (N,N-Diethyl-Phenylene Diamine) procedure. This method is a nonspecific test that simply detects the presence of any oxidizing agents in the sample. A more specific colorimetric test that is used by these devices involves the reaction of monochloramine with an alkaline solution of a phenol to form a type of Indophenol Blue dye. Again, in these measurement systems, there are several solutions that would have to be replenished from time to time, as well as periodic replacement of pump tubing and other expendable parts. Furthermore, a very large dilution would be necessary for some oxidant chemistries of commercial interest in order to reach the operating range of the measurement devices. This dilution can introduce serious errors in the measurement. Further, it would not be possible to monitor small variations in a total oxidant level, where these dilution errors arise and become problematic. In addition, these colorimetric devices can have a high unit cost. Further, on-line colorimetric equipment can require pumps to keep a portion of fluid flowing through optical cells, which can involve additional moving parts which can be subject to failure.
Polarographic measurements also have been investigated, which use platinum and glassy carbon electrodes. Reproducibility and signal drift have been found to be serious problems, rendering this approach unfeasible. Also, platinum electrodes are oxidized by halogen-containing compounds, such as halogen-containing oxidants, which makes them an undesirable choice for use with those chemistries.
The present investigators have recognized that there is a need for techniques and equipment for measurement of the total quantities of an electrolytically-active species in samples of aqueous solution to provide rapid and accurate analyses of species concentrations in samples of process fluids, which can be used to improve process control and avoid or reduce the indicated drawbacks of other analytical techniques and equipment used therefor.